Session: Government Agency Student Posters

Paper Number: 172977

Heterobarrier In-Situ Phonon Recycling in Semiconductor Diodes 

The energy efficiency of semiconductor devices, such as light-emitting diodes (LEDs), solar cells, or laser diodes, is limited by the undesirable conversion of electricity into waste heat. For example, LEDs typically convert electricity into light with less than 50% efficiency, while the remaining electric power turns into heat. Most of this heat originates from atomic vibrations, i.e., phonons emitted during electron thermalization or nonradiative decay. Recovering this waste heat back to electric power presents an opportunity for significant device efficiency gains. In this research, we investigate an innovative semiconductor heterostructure designed to recycle phonons in-situ within diode devices, targeting an improvement in energy efficiency and thermal performance.

In semiconductor diodes, thermionic transport is relevant to the carrier (electron and hole) kinetics, particularly under low forward bias. Specifically, the depletion region generates a built-in potential barrier junction that restricts the transport of low-energy carriers. Meanwhile, carriers with sufficient kinetic energy can transmit over the potential barrier and contribute to current. In compound semiconductors (e.g., GaAs, GaN), the remaining unextracted low-energy carrier population upstream of the junction absorbs polar optical phonons to restore the equilibrium carrier distribution. This is also known as the Peltier effect.

In this research, we introduce a thin, alloy-graded, n-type semiconductor heterobarrier layer and demonstrate that it enhances thermionic transport in GaN diodes, thus increasing the electron current and upstream phonon absorption by conduction electrons. The band structure and effective mass in the heterobarrier region is tuned by the composition of AlInGaN quaternary alloying. The Al alloying is used to create an additional small potential barrier, while In alloying is added to grade the potential back to the bulk GaN bandgap at the end of the layer. The quaternary-alloyed composition allows the effective mass in the layer to remain larger than bulk GaN, and this characteristic causes a larger number of energized electrons to contribute to current.

The design characteristics and transport physics of the proposed design are investigated by the self-consistent ensemble Monte Carlo (MC) method. The MC method tracks thousands of individual electron trajectories involving scattering with phonons (optical, acoustic), impurities, and alloy (Al, In) atoms. The electrostatics (electric potential distribution) are evaluated self-consistently by solving the Poisson equation with the updated carrier density at every timestep. The MC simulation results demonstrate the predicted current gain effect. At low applied bias, we show that electron current in the heterobarrier-integrated diode increases by up to 10% compared to the same device with no heterobarrier. Under increasing applied bias, efficiency falls due to electrostatic and nonequilibrium transport effects, which are elucidated by statistical MC data. In a sample GaN LED operating at room temperature, a 10% current gain by phonon absorption would extract 15% of the phonons emitted by nonradiative decay, suggesting a significant reduction in thermal load while simultaneously improving the electrical performance.

Presenting Author: Lorenzo Franceschetti University of Michigan

Presenting Author Biography: Lorenzo Franceschetti is a PhD candidate in Mechanical Engineering at the University of Michigan. His research uses multiscale, computational and theoretical approaches to investigate carrier transport in semiconductor devices. Specifically, he is exploring innovative methods to recycle waste heat in modern energy technologies, such as light-emitting diodes (LEDs). In 2024, he received the NSF Graduate Research Fellowship (GRF).

Lorenzo earned a Bachelor of Science in Aerospace Engineering from the University of Tennessee in 2023. His previous research focused on high-temperature thermal management. As an intern at a nuclear company, he worked on energy transport and storage technology.

Authors:

Lorenzo Franceschetti University of Michigan
Seungha Shin University of Tennessee
Massoud Kaviany University of Michigan